Extended tuning in external cavity quantum cascade lasers

Abstract

In a semiconductor lasers using quantum well gain medium, a quantum well stack is mounted in an epi-down configuration. The epitaxial side of the device may be directly bonded to an efficient heat transport system so that heat may more easily leave the quantum well stack layers and be disposed at a heatsink. Such a device runs cooler and exhibits reduced loss mechanisms as represented by a laser system loss-line. External cavity systems using this configuration may permit a high degree of tunability, and these systems are particularly improved as the tuning range is extended by lowered cavity losses.

1. An optical source comprising:
 a quantum well gain medium; 
 a thermally conductive substrate; and 
 an optical resonator, 
 wherein said quantum well gain medium has a base side and an epi side, and is bonded to the thermally conductive substrate on the epi side, and 
 wherein said optical resonator is less than 30 millimeters is length and comprises at least one optical feedback element spatially removed from the quantum well gain medium, 
 wherein said quantum well gain medium further comprises: a quantum well stack; waveguide layers; regrowth material; emission facets; and a bond layer, 
 wherein said quantum well stack is comprised of a plurality of semiconductor layers forming quantum wells and barrier regions, 
 wherein said quantum well gain medium bonded to said thermally conductive substrate forms an electrically conductive serial circuit whereby electrical current may pass from the base side, through the quantum wells, further through the bonded surface, and into the bond layer, and 
 wherein said waveguide layers are disposed on either of two opposing sides of the quantum well stack and are optically and electrically conductive, 
 wherein said regrowth material is electrically insulative, optically opaque, and disposed on either of two opposing sides of the quantum well stack, 
 wherein said emission facets are planar surfaces formed in a natural plane of the quantum well stack operable for coupling of optical beams into and out of the gain medium, and 
 wherein said bond layer is a metallic or electrically conductive layer disposed on a waveguide layer to form a bond between the epi side of the quantum well gain medium and the thermally conductive substrate. 
 
     
     
       2. An optical source as claimed in  claim 1 , wherein said quantum well gain medium is an epitaxially grown multi-layer quantum well stack formed on a crystal base, and
 wherein said optical feedback elements comprise two opposing elements arranged to couple a feedback optical beam back into said quantum well gain medium from which an amplified beam arises thus forming an optical resonant cavity. 
 
     
     
       3. An optical source as claimed in  claim 2 , wherein said two opposing elements are separated by no more than about 30 millimeters. 
     
     
       4. An optical source as claimed in  claim 1 , further comprising a first bonding pad in electrical contact with the bond layer suitable for wire bond attachments a second bonding pad in electrical contact with the base side of the quantum well gain medium. 
     
     
       5. An optical source as claimed in  claim 4 , wherein said bonding pads are arranged to support wire bond connections, said wire bond connections being suitable for high pulsed current applications. 
     
     
       6. An optical source as claimed in  claim 1 , wherein at least one of said facets is prepared with an anti-reflective coating to increase coupling between the gain medium and free space. 
     
     
       7. An optical source as claimed in  claim 6 , wherein said prepared facet is coupled to a spatially removed optical feedback element. 
     
     
       8. An optical source as claimed in  claim 7 , wherein said prepared facet is coupled to the optical feedback element via a lens having a short focal length and high numerical aperture, the lens arranged between the prepared facet and the optical feedback element the lens planar side towards the prepared facet and the convex side toward the feedback element such that substantially collimated light falls incident upon the feedback element. 
     
     
       9. An optical source as claimed in  claim 8 , wherein said feedback element is a wavelength select element that feeds back a selected wavelength to the gain medium and discriminates against other wavelengths. 
     
     
       10. An optical source as claimed in  claim 9 , wherein said wavelength select element is static. 
     
     
       11. An optical source as claimed in  claim 9 , wherein said wavelength select element is a dynamic or tunable wavelength select element whereby the wavelength fed back to the gain medium is changeable. 
     
     
       12. An optical source as claimed in  claim 11 , wherein said wavelength select element is a grating mounted on a pivot axis. 
     
     
       13. An optical source as claimed in  claim 11 , wherein said wavelength select element is a prism. 
     
     
       14. An optical source as claimed in  claim 12 , wherein said grating is further coupled to an electromechanical actuator arranged to move the grating about a pivot axis in response to an applied electronic signal. 
     
     
       15. An optical source as claimed in  claim 1 , wherein said thermally conductive substrate is diamond. 
     
     
       16. An optical source comprising:
 a quantum well gain medium; 
 a thermally conductive substrate; and 
 an optical resonator, 
 wherein said quantum well gain medium has a base side and an epi side, and is bonded to the thermally conductive substrate on the epi side, 
 wherein said optical resonator is less than 30 millimeters is length and comprises at least one optical feedback element spatially removed from the quantum well gain medium, 
 wherein said thermally conductive substrate is formed of a bulk substrate portion and a carrier substrate portion thermally coupled to the bulk substrate portion, 
 wherein said bulk substrate portion has a thermal conductivity greater than about 100 W/mK, and 
 wherein said carrier substrate portion is formed of material having thermal conductivity greater than about 500 W/mK and is substantially smaller than the bulk substrate portion. 
 
     
     
       17. An optical source as claimed in  claim 1 , wherein the thermally conductive substrate further includes a surface suitable for coupling to a thermoelectric cooling TEC system. 
     
     
       18. An optical source as claimed in  claim 17 , wherein said thermally conductive body is characterized as one having a thermal conductivity greater than about 200 W/mK. 
     
     
       19. An optical source as claimed in  claim 18 , wherein said thermal conductivity is between about 1500 and 2000 W/mK. 
     
     
       20. An optical source as claimed in  claim 19 , wherein said thermally conductive body is diamond.